1. Field of the Invention
The present invention relates to a construction machine including first, second and third pumps and a hydraulic circuit including first, second and third circuits corresponding to the pumps, respectively.
2. Description of the Background Art
A background art of the present invention is described, taking a hydraulic excavator shown in FIG. 6 as an example.
This hydraulic excavator includes a crawler-type base carrier 1, an upper slewing body 2 mounted on the base carrier 1 so as to be slewable about a vertical axis X, and a working attachment 9 attached to the upper slewing body 2, the working attachment 9 including a boom 3, an arm 4 and a bucket 5. Furthermore, the hydraulic excavator includes, as hydraulic actuators, a boom cylinder 6 for raising and lowering the boom 3, an arm cylinder 7 for rotating the arm 4, a bucket cylinder 8 for rotating the bucket 5, right and left travel motors for causing the base carrier 1 to travel by driving respective right and left crawlers included in the base carrier 1, and a slewing motor for slewing the upper slewing body 2.
Such a hydraulic excavator is provided with a hydraulic circuit to drive each of the actuators. For example, there is known a hydraulic circuit having i) a first circuit including one of the right and left travel motors and the boom cylinder 6, ii) a second circuit including the other travel motor and the arm cylinder 7 and iii) a third circuit including the slewing motor, the first to third circuits being connected to respective first to third pumps, as disclosed in Japanese Patent No. 3681833.
In such a hydraulic circuit, a flow combining valve is often provided to enable a boom to be quickly raised when a boom-raising/slewing operation is performed. The boom-raising/slewing operation is a composite operation to simultaneously perform the boom raising movement of raising the boom and a slewing movement of slewing the upper slewing body. This flow combining valve, which is for switching a fluid path for hydraulic fluid discharged by the third pump, has a first position, that is, a neutral position, and a second position, adapted to be switched from the first position to the second position when the boom raising/slewing operation is performed. When switched to the second position, the flow combining valve forms a fluid path for supplying third pump fluid, which is hydraulic fluid discharged by the third pump, to the boom cylinder in parallel to the slewing motor, that is, for combining the third pump fluid with first pump fluid which is hydraulic fluid discharged from the first pump into a combined flow.
However, the flow combining valve has a response delay from the start of performing the boom raising/slewing operation by an operator until the position of the flow combining valve is actually switched from the first position to the second position, the response delay causing a time lag which may give a shock to the slewing movement. For example, in the case of starting the boom raising operation during a performance of the slewing operation, if the flow combining valve was switched from the first position to the second position simultaneously with the start of the boom raising operation, a maximum pressure (slewing pressure) of the slewing motor would be gradually reduced according to the boom raising operation with no shock; however, in actual, the flow combining valve is actually switched to the second position with a certain delay from the start of the boom raising operation (i.e. the flow combining valve is switched to the second position in a state where the boom raising operation has progressed to a certain extent), thus causing a state of the supply of the third pump to be suddenly switched from a first state where the third pump fluid is supplied only to the slewing motor to a second state where the third pump fluid is supplied to the slewing motor and the boom cylinder in parallel. The sudden switch of the supply state of the third pump fluid involves sharp change in the maximum pressure (slewing pressure) of the slewing motor from a relief pressure to a boom operating pressure, thereby generating a possibility of applying a notable shock to the slewing movement. Such a shock can cause a reduction in operability. On the other hand, there is also a demand to ensure a sufficient force for driving the arm during a horizontal attraction operation based on a combination of the boom raising movement and an arm attracting movement, i.e. a movement of the arm in a attracting direction.